Like Batman, he wouldn't kill because he believed in his fathers philosophical view on life. That life, no matter who's it is, ALL LIFE is sacred, and should be saved and preserved by whatever means. That's why Batman saves the lives of even the Joker. Because even HIS life is sacred. He hates those who take life, but it's not within him to break his views on life, reinforced by his belief that he is a good person, and all good people should do the right thing...to preserve life, not end it.

Yes, Nolan (for one) clearly drew that line. In BB, Bruce says he’s “no executioner.” And in TDK, Batman can’t bring himself to kill the Joker in cold blood. (Indeed, as you mention, he actually saves his life later.) That said, there were the inadvertent deaths of Ducard and Dent. Now it doesn’t seem that those were treated as egregious violations of Batman’s “code” (note, again, the “inadvertent” proviso). Yet, if a similar circumstance were to occur in a Supes story, there would be (at least some) strenuous objections – “No! They obviously don’t get Superman.”

But if no moral denunciations apply to the Batman example, I’m puzzled as to why they would in the (hypothetical) Superman case.